Nascent major histocompatibility complex (MHC) class I molecules undergo a stringent maturation process within the endoplasmic reticulum (ER) of cells, culminating in the cell-surface presentation of endogenous and viral peptides to cytotoxic T-cells (CTLs). The importance of this pathway in controlling viral infections is underlined by the fact that viruses have evolved numerous strategies to interfere with MHC I maturation and suppress antigen presentation. It is thought that these strategies enable viruses to persist in infected host cells. Because persistent viruses are clinically relevant pathogens, itis essential that we understand how they evade detection by the human immune system. This knowledge is relevant for understanding host-pathogen interaction, and it also informs us on the susceptibility of the class I antigen presentation pathway to viral interferences. Our ability to harness this information can benefit the development of antiviral drugs. Human Adenoviruses (Ads) cause diseases ranging from gastrointestinal and respiratory tract infections to eye infections. Ad encodes the E3-19K protein that binds to and retains MHC class I molecules within the ER of infected cells. It was shown that E3-19K reduces levels of MHC I expression on infected cells, an effect that protects infected cells from lysis by CTLs. We have been interested in studying the molecular mechanism by which E3-19K evades immune surveillance for many years. We have recently made a number of key observations on E3-19K function. We showed that Ad serotype 2 (Ad2) E3-19K associates with immature (peptide-free) and mature (peptide-filled) MHC I molecules. We also showed that E3-19K targets MHC I molecules of the HLA-A and -B loci but not those of the HLA-C and -E loci. Importantly, we determined the first three-dimensional structure of an E3-19K/MHC I complex, namely Ad2 E3-19K bound to HLA-A2, by x-ray crystallography. To date, many important aspects of the immunomodulatory function of E3-19K remains incompletely understood. It is not known if E3-19K utilizes more than one strategy to interfere with MHC I maturation within the ER (Aim #1); it is also not known which MHC I molecules are specifically downregulated by E3-19K on infected cells (Aim #2); no one has yet examined the role of the E3-19K/MHC I association on natural killer functions (Aim #2); and we still lack an understanding of the structural basis for high affinity and specificity of interaction in the E3-19K/MHC I association (Aim #3). The goal of this project is to build on our previous contributions and address these salient questions in a systematic manner. The significance of our studies is highlighted by the critical role that the class I antigen presentatin pathway plays in eliminating invading pathogens, the novelty of our Ad2 E3-19K/HLA-A2 structure, and the knowledge that Ad infections can be fatal in children and immunocompromised patients. We expect that upon completing our studies, we will have a more in depth understanding of how E3-19K counteracts MHC I- restricted cellular immune defenses and influences the course of Ad infection.